Edge emitting semiconductors lasers for high output power are usually embodied as broad stripe lasers in which the active region can have a width of, for example, 100 μm or more. Owing to the comparatively large lateral extent of the active region, in general a large number of lateral laser modes can commence oscillation in the case of semiconductor lasers of this type. Multimode operation of an edge emitting semiconductor laser makes it more difficult, in particular, to couple the emitted laser light into downstream optical elements, for example, into optical waveguides.
To suppress higher lateral laser modes, in particular to obtain operation in the lateral fundamental mode, WO 01/97349 A1 discloses forming phase structures in the waveguide of an edge emitting semiconductor laser. The phase structures are regions of the semiconductor body in which the effective refractive index deviates from the effective refractive index of the regions of the semiconductor body that adjoin in a lateral direction, and which are embodied such that higher laser modes incur greater circulation losses in the laser resonator than the lateral fundamental mode of the semiconductor laser. The phase structures can be produced in the edge emitting semiconductor laser, for example, by structures being etched into the semiconductor body from the surface of the semiconductor body, which structures extend into the second cladding layer or even into the waveguide region. The structures can be optimized, for example, by simulations such that they produce lower losses for the lateral fundamental mode than for the higher laser modes such that the oscillation build-up of the laser in the lateral fundamental mode is promoted.
The phase structures known per se for semiconductor lasers that emit in the visible spectral range cannot readily be transferred to semiconductor lasers that emit in the infrared spectral range. This is due to the fact that the laser radiation in the case of semiconductor lasers in the infrared spectral range is concentrated onto the waveguide region to a greater extent and penetrates into the cladding layers to a lesser extent than the laser radiation in the case of semiconductor lasers in the visible spectral range. Moreover, the cladding layers in the case of semiconductor lasers in the infrared spectral range are comparatively thick on account of the comparatively high wavelength. For this reason, the effect of phase structures having small etching depths in the second cladding layer is only very small. On the other hand, a great dependence of the effect of phase structures on the etching depth is manifested if etching is effected into the vicinity of the waveguide region. To realize phase structures for semiconductor lasers in the infrared spectral range, therefore, it would be necessary to produce comparatively deep etching structures in the semiconductor body, the depths of which are very precisely defined. However, both requirements can be simultaneously fulfilled only with difficulty.
It could therefore be helpful to provide an edge emitting semiconductor laser comprising a semiconductor layer sequence that simplifies the production of phase structures such that phase structures can, in particular, also be realized for semiconductor lasers in the infrared spectral range.